lkk,
when you say the "balance" is gone, do you mean stereo image balance?--cause that's what usually "balance" means. (and that does get screwed up at low volumes too because some cheap potentiometers do not have their channels matched well.) but if you're only talking about the bass, it's as ooheadsoo said, the lower the volume, the less bass you will hear. and that's not a crossover issue. anyway, even if your crossover was setup wrong, you would still get the same frequency "balance" at any volume--so the bass will sound smaller at all volumes. but anyway, no, your crossover ain't broke... blame your woes on God--he made your ears.
you have two choices: either get used to no bass at low volumes, or use an EQ.
(ps--that's one reason why people like loud music... you hear a fuller representation of the music. more bass. more sound. just sounds better... but again, God made your ears like dissposable tissue--so you can't listen to music properly for too long.)